1. Field of the Invention
This invention relates generally to the design of magnetic recording heads for high recording density, particularly heads designed for perpendicular magnetic recording.
2. Description of the Related Art
The commonly used inductive magnetic write head technology utilizes the longitudinal configuration. In this configuration a magnetic field fringes across a write gap between an upper and a lower pole in the plane of the air bearing surface (ABS) of the write head. This plane will be referred to as the longitudinal plane. The ABS is positioned above and parallel to the surface of a recording medium, such as a hard disk. The surface of the hard disk is formed of a magnetic medium. As the hard disk moves beneath the ABS, a horizontal component of the fringing field of the write head interacts with and aligns regions of sharp magnetic transitions (essentially magnetic domains) which are horizontally disposed within the surface of the magnetic medium.
Perpendicular magnetic recording (PMR) offers a new writing configuration that is a viable replacement candidate for the horizontal configuration as recording area densities approach 500 Gb/in2. At this ultra-high area density, the super-paramagnetic limit of magnetic media becomes a problem, which is the condition wherein magnetic domains are so small that they lack thermal stability and can be randomly aligned by thermal agitation.
As is well explained by Tanaka et al. (U.S. Pat. No. 6,128,166) and also by Batra et al. (U.S. Patent Application Publication No.: US 2002/0071208 A1), in perpendicular recording, the magnetic recording medium is formed in two layers, an upper layer formed vertically over a lower layer. The lower layer is formed of soft magnetic material. An upper layer has a high coercivity (it is a hard magnetic material) and a vertical magnetic anisotropy, ie an anisotropy that is perpendicular to the surface plane of the recording medium. The soft magnetic lower layer acts to concentrate the field produced by the write head. This configuration has the potential to support much higher recording densities due to a reduced demagnetization field required to induce sharp magnetic transitions during the recording process. The fringing field across the write gap of the write head then interacts with the vertical anisotropy of the upper layer with a perpendicular field component and its gradient, rather than a horizontal component, and produces sharp transitions therein. The vertical transitions in the perpendicular recording medium are more stable thermally due, in part, to a thicker upper layer, so the super-paramagnetic limits is no longer as significant a problem it is in longitudinally anisotropic media.
The design of a perpendicular magnetic recording (PMR) head offers new challenges, since it must provide a writing field of extremely high definition and sharp field gradient, compatible with the increased area density of the medium and its correspondingly narrower track widths. Batra et al. (cited above) shows the basic design and operation of a perpendicular write head of the prior art, which is shown also in our FIG. 1. Referring to FIG. 1, there is shown schematically a side cross-sectional view of a particularly simplified write head (10), its magnetic field (200) and a magnetic medium moving beneath it. The magnetic medium has two layers, a lower soft layer (210) and an upper hard layer (300) with vertically oriented magnetic domains (arrows (45)). The medium is moving from right to left, as shown by the arrow (55). The pole structure of the write head includes a return (lower) pole (15) and a main writing (upper) pole (17) with a gap (19) between them. An induction coil (60) is wound around the pole to produce the magnetic field (shown by closed field lines (200). The field emerges from the main pole and returns through the lower pole. Batra et al. note that the write head illustrated will create problems of unwanted side writing because of the lack of a shielding mechanism to contain the field laterally and prevent the field from spreading beyond the track being written upon. Batra, therefore, teaches a write head in which there are two return poles and a central write pole formed between them, wherein side shields are formed on either side of the poles.
More traditional longitudinal write heads have also had to deal with problems associated with narrow tracks, such as unwanted side writing (writing on tracks adjacent to the actual track being written on). One approach is taught by Chang et al. (U.S. Pat. No. 6,278,591 B1), which is to form the pole with an inverted head. Das (U.S. Pat. No. 5,075,956) teaches a write head in which the pole tip is surrounded on either side by shields that contain unwanted flux spreading. Another approach to producing such write heads with narrow pole tips and correspondingly highly defined write gaps has been the stitched pole write head. This design permits the pole tip to be formed separately, whereby its shape and dimensions can be carefully controlled, then “stitched” or plated onto a larger pole piece. Chen et al. (U.S. Pat. No. 6,591,480 B1) teaches a process for forming a stitched longitudinal write head with a narrow pole tip in which the magnetic flux across the write gap is concentrated by a dielectric-filled gap in the lower pole piece.
The purpose of the present invention is to provide a novel shield formation for a PMR head that effectively eliminates adjacent track erasure (ATE) by concentrating the writing field at the position of the track being written upon and effectively shielding the writing field from leakage into regions laterally displaced from the track being written upon. More specifically, we propose a write shield in which a small additional shield is stitched onto a main write shield, between the main write shield and the write gap of the head. Not only does this shield serve to prevent ATE, but it improves the vertical field gradient for more efficient writing.